System Threads

A system thread is a thread that the kernel spawns automatically during system initialization.

The kernel spawns the following system threads:

Main thread

This thread performs kernel initialization, then calls the application’s main() function (if one is defined).

By default, the main thread uses the highest configured preemptible thread priority (i.e. 0). If the kernel is not configured to support preemptible threads, the main thread uses the lowest configured cooperative thread priority (i.e. -1).

The main thread is an essential thread while it is performing kernel initialization or executing the application’s main() function; this means a fatal system error is raised if the thread aborts. If main() is not defined, or if it executes and then does a normal return, the main thread terminates normally and no error is raised.

Idle thread

This thread executes when there is no other work for the system to do. If possible, the idle thread activates the board’s power management support to save power; otherwise, the idle thread simply performs a “do nothing” loop. The idle thread remains in existence as long as the system is running and never terminates.

The idle thread always uses the lowest configured thread priority. If this makes it a cooperative thread, the idle thread repeatedly yields the CPU to allow the application’s other threads to run when they need to.

The idle thread is an essential thread, which means a fatal system error is raised if the thread aborts.

Additional system threads may also be spawned, depending on the kernel and board configuration options specified by the application. For example, enabling the system workqueue spawns a system thread that services the work items submitted to it. (See Workqueue Threads.)

Implementation

Writing a main() function

An application-supplied main() function begins executing once kernel initialization is complete. The kernel does not pass any arguments to the function, unless CONFIG_BOOTARGS is selected. In such case the kernel passes arguments to it and main(int, char **) can be used.

The following code outlines a trivial main(void) function. The function used by a real application can be as complex as needed.

int main(void)
{
    /* initialize a semaphore */
    ...

    /* register an ISR that gives the semaphore */
    ...

    /* monitor the semaphore forever */
    while (1) {
        /* wait for the semaphore to be given by the ISR */
        ...
        /* do whatever processing is now needed */
        ...
    }
}

Suggested Uses

Use the main thread to perform thread-based processing in an application that only requires a single thread, rather than defining an additional application-specific thread.